Fuel cell gas diffusion layer coating process and treated article

ABSTRACT

A method is provided for making a hydrophobic carbon fiber construction, such as a fuel cell gas diffusion layer, by electrophoretic deposition of a highly fluorinated polymer, which may be followed by sintering of the fluoropolymer. A hydrophobic carbon fiber construction is provided, such as a fuel cell gas diffusion layer, which is coated with a monolayer of particles of a highly fluorinated polymer, which may be sintered.

FIELD OF THE INVENTION

[0001] This invention relates to a method of making a hydrophobic carbonfiber construction such as a fuel cell gas diffusion layer byelectrophoretic deposition of a highly fluorinated polymer which may befollowed by sintering of the fluoropolymer. This invention additionallyrelates to a hydrophobic carbon fiber construction coated with amonolayer of particles of a highly fluorinated polymer, which may besintered.

BACKGROUND OF THE INVENTION

[0002] Watanabe, “improvement of the Performance and Durability of Anodefor Direct Methanol Fuel Cells,” Proceedings of the Workshop on DirectMethanol-Air Fuel Cells, pp. 24-36 (1992), discloses a method ofwet-proofing which involves coating carbon black with polyethylene outof a polyethylene latex, perfluorinating the polyenthylene in situ onthe surface of the carbon black, and coating a gas diffusion layer withthe hydrophobic carbon black.

[0003] U.S. Pat. No. 6,080,504 discloses a method of electrodepositionof catalytic metal on a substrate to form a gas diffusion electrodeusing a pulsed electric current.

[0004] U.S. Pat. No. 5,298,348 and 5,389,471 disclose a seperator for analkaline battery system.

[0005] U.S. Pat. No. 6,083,638 discloses a fuel cell system thatincludes a current collector that includes hydrophilic materials and canalso include hydrophobic materials. The current collector may be made offibers such as carbon, glass or resin fibers. The hydrophilic materialor bulking agent may be particles of materials such as carbon powder,metal powder, glass powder, ceramic powder, silica gel, zeolite ornon-fluorinated resin. The hydrophobic material or bulking agent may beparticles of materials such as fluorinated resin. (see, '638 FIG. 10).

[0006] U.S. Pat. No. 5,998,058 discloses an electrode backing layer fora polymer electrolyte membrane fuel cell formed from a carbon fibersubstrate treated so as to contain both “hydrophilic” and “hydrophobic”pores. The reference describes a method of making pores more hydrophilicby immersion in a solution of tin tetrachloride pentahydrate followed byimmersion in ammonia.

[0007] U.S. Pat. No. 6,024,848 discloses a porous support plate for anelectrochemical cell which includes a contact bilayer adjacent to anelectrode including a hydrophobic and a hydrophilic phase. The referencediscloses a hydrophilic phase comprised of a mixture of carbon black anda proton exchange resin.

SUMMARY OF THE INVENTION

[0008] Briefly, the present invention provides a method of making ahydrophobic carbon fiber construction such as a fuel cell gas diffusionlayer comprising the steps of: a) immersing a carbon fiber constructionin an aqueous dispersion of a highly fluorinated polymer, typically aperfluorinated polymer; b) contacting the dispersion with acounterelectrode; and c) electrophoretically depositing the highlyfluorinated polymer onto the carbon fiber construction by applyingelectric current between the carbon fiber construction and thecounterelectrode. Typically the carbon fiber construction is the anodeand the counterelectrode is the cathode. Typically a voltage of greaterthan 6 volts is applied. Typically the step of electrophoreticallydepositing the highly fluorinated polymer can be accomplished in 30minutes or less, more typically 15 minutes or less.

[0009] In another aspect, the present invention provides hydrophobiccarbon fiber construction made according to the electrophoretic methodof the present invention, in particular one having a highly uniformcoating of a highly fluorinated polymer.

[0010] In another aspect, the present invention provides a hydrophobiccarbon fiber construction coated with a monolayer of particles of ahighly fluorinated polymer. In a further embodiment, the particles ofhighly fluorinated polymer may be sintered.

[0011] What has not been described in the art, and is provided by thepresent invention, is a method of manufacturing a hydrophobic gasdiffusion layer for use in a fuel cell by electrophoretic deposition ofa fluoropolymer.

[0012] In this application:

[0013] “monolayer” typically refers to a layer of particles on a surfacethat has a depth of not more than one particle over substantially all ofthe surface, and may optionally include a layer grown to a thicker depththan one particle if substantially all of the surface has first beencovered with a layer of abutting particles having a depth of oneparticle; and

[0014] “highly fluorinated” means containing fluorine in an amount of 40wt % or more, but typically 50 wt % or more, and more typically 60 wt %or more.

[0015] It is an advantage of the present invention to provide a quickand convenient method of manufacturing a hydrophobic gas diffusion layerhaving a uniform coating of a fluoropolymer.

BRIEF DESCRIPTION OF THE DRAWING

[0016]FIG. 1 is an electron micrographs of a fluoropolymer-coatedsubstrate according to the present invention at 11,600× magnification.

[0017]FIG. 2 is an electron micrographs of a fluoropolymer-coatedsubstrate according to the present invention at 5,800× magnification.

[0018]FIG. 3 is an electron micrographs of a comparativefluoropolymer-coated substrate at 1,990× magnification.

[0019]FIG. 4 is an electron micrographs of a comparativefluoropolymer-coated substrate at 9,200× magnification.

[0020]FIG. 5 is an electron micrographs of a fluoropolymer-coatedsubstrate according to the present invention at 3,500× magnification.

[0021]FIG. 6 is an electron micrographs of a fluoropolymer-coatedsubstrate according to the present invention at 3,100× magnification.

[0022]FIG. 7 is a graph of data showing resistivity vs. compression forcarbon papers treated according to the present invention (2 and 3) and acomparative untreated paper (1).

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

[0023] The present invention provides an electrophoretic method ofmaking a hydrophobic carbon fiber construction such as a fuel cell gasdiffusion layer. Briefly, the present method comprises the steps of: a)immersing a carbon fiber construction in an aqueous dispersion of ahighly fluorinated polymer; b) contacting the dispersion with acounterelectrode; and c) electrophoretically depositing the highlyfluorinated polymer onto the carbon fiber construction by applyingelectric current between the carbon fiber construction and thecounterelectrode.

[0024] Fuel cells are electrochemical cells which produce usableelecticity by the catalyzed combination of a fuel such as hydrogen andan oxidant such as oxygen. Typical fuel cells contain layers known asgas diffusion layers or diffuser/current collector layers adjacent tocatalytically reactive sites. These layers must be electricallyconductive yet must be able to allow the passage of reactant and productfluids. Typical gas difusion layers comprise porous carbon materials. Insome fuel cell systems, it is advantageous to use a gas diffusion layerwhich is more hydrophobic than untreated carbon. The present inventionconcerns the manufacture of hydrophobic gas diffusion layers.

[0025] Any suitable carbon fiber construction may be used. Typically thecarbon fiber construction is selected from woven and non-woven carbonfiber constructions. Carbon fiber constructions which may be useful inthe practice of the present invention may include: Toray™ Carbon Paper,SpectraCarb™ Carbon Paper, AFNTM non-woven carbon cloth, Zoltek™ CarbonCloth, and the like.

[0026] Any suitable electrodeposition equipment may be used, including aHull Cell. Typically the carbon fiber construction is the anode and thecounterelectrode is the cathode. A typical counterelectrode is mildsteel plate. Any suitable source of electric current may be used.

[0027] Any suitable aqueous dispersion of highly fluorinated polymer maybe used. The dispersion may be a colloidal suspension or a latex.Average particle size in the dispersion is typically less than 500 nmand more typically between 300 and 50 nm. The highly fluorinated polymeris typically a perfluorinated polymer, such as polytetrafluoroethylene(PTFE), fluorinated ethylene propylene (FEP), perfluoroalkyl acrylates,hexafluoropropylene copolymers,tetrafluoroethylene/hexafluoropropylene/vinylidene fluoride terpolymers,and the like.

[0028] The electric current applied between the carbon fiberconstruction and the counterelectrode is sufficient to deposit thedesired amount of fluoropolymer. Typically, the electric current isapplied at a voltage of at least 6 volts, more typically at least 15volts, and most typically at least 30 volts. However it is an advantageof the present method that it can be performed using relatively lowvoltages of less than 100 volts and more typically less than 50 volts.

[0029] It is an advantage of the present method that it can be performedin a speedy manner and is therefore suitable for commercial production.Typically, the duration of the electrodeposition step is not more than30 minutes, more typically not more than 15 minutes.

[0030] Typically the highly fluorinated polymer is deposited onto thecarbon fiber construction in the amount of at least 0.1 weight percentper weight of carbon fiber construction, more typically at least 1weight percent, more typically 1 to 10 weight percent, and mosttypically 1 to 5 weight percent. Higher levels of deposition from 5 to30 weight percent or more may also be achieved.

[0031] Typically, the treated carbon fiber construction is subsequentlyrinsed and dried.

[0032] The treated carbon fiber construction may also be heated tosinter the fluoropolymer particles. Sintering temperatures depend on thefluoroplymer chosen, but are typically at least 150° C., more typicallyat least 250° C., and most typically at least 350° C. Sintering time istypically at least 10 minutes, more typically at least 20 minutes, andmost typically at least 30 minutes. Additionally, coatings may be addedincluding hydrophobic coatings such as fluoropolymer/carbon coatings.

[0033] Fluropolymer coatings produced according to the method of thepresent invention are uniquely uniform. FIGS. 1, 2, 5 and 6 aremicrographs of substrates coated according to the present invention. Itcan bee seen that the particles of fluoropolymer form a monolayer on thesurface of the fibers. In contrast, the comparative fluoropolymer-coatedsubstrates appearing in FIGS. 3 and 4 contain clumped fluoropolymerparticles. FIG. 3 illustrates that fluoropolymer particles tend toconcentrate at the intersections of fibers in the course of thecomparative dipping and drying method. Large areas of many fibers areentirely uncoated. Without wishing to be bound by theory, it is believedthat the method according to the present invention forces a uniformdistribution of fluoropolymer because of the insulating nature of thecoating.

[0034] This invention is useful in the manufacture of hydrophobic fuelcell gas diffusion layers.

[0035] Objects and advantages of this invention are further illustratedby the following examples, but the particular materials and amountsthereof recited in these examples, as well as other conditions anddetails, should not be construed to unduly limit this invention.

EXAMPLES

[0036] Unless otherwise noted, all reagents were obtained or areavailable from Aldrich Chemical Co., Milwaukee, Wis., or may besynthesized by known methods.

Examples 1 and 2C

[0037] In Example 1, Teflon® PTFE 30B colloidal suspension (DuPontFluoroproducts, Wilmington, Del.) was electrodeposited on Toray™ CarbonPaper 060 (Toray International Inc., Tokyo, Japan). A 1 cm² piece ofcarbon paper was used as the anode of the electrolytic cell and a mildsteel plate was used as the cathode. The PTFE suspension was diluted to1% by weight with deionized water. A 6 volt potential was appliedbetween the anode and cathode for 15 minutes to deposit the PTFEparticles on the carbon paper. The sample was dried.

[0038] In Comparative Example 2C, Toray™ Carbon Paper 060 (TorayInternational Inc., Tokyo, Japan) was dipped in the same 1% Teflon® PTFE30B colloidal suspension for 15 minutes and dried.

[0039]FIGS. 1 and 2 are electron micrographs of the coated product ofExample 1. FIGS. 3 and 4 are electron micrographs of the coated productof Comparative Example 2C. These micrographs demonstrate the high degreeof uniformity obtained by use of the method according to the presentinvention.

Example 3 and 4

[0040] In Examples 3 and 4, Teflon® PTFE 30B colloidal suspension(DuPont Fluoroproducts, Wilmington, Del.) was diluted to 1% by weightwith deionized water and poured into a Hull Cell. Toray™ Carbon Paper060 (Toray International Inc., Tokyo, Japan) was fitted into the HullCell as the anode. The cathode was mild steel. The electrode distancewas 40 mm. Nominal surface area of cathode was 33 cm² and anode was 28cm². For Example 3, a 15 volt potential was applied between the anodeand cathode for 15 minutes to deposit the PTFE particles on the carbonpaper. For Example 4, a 30 volt potential was applied between the anodeand cathode for 15 minutes to deposit the PTFE particles on the carbonpaper. The carbon paper was removed and gently rinsed in DI water. Thesample was dried in air for 1 hour, pumped down under vacuum and imagedunder an electron microscope to observe the deposition progress.

[0041]FIGS. 5 and 6 are electron micrographs of the coated products ofExamples 3 and 4, respectively. The micrographs demonstrate thatuniformity and density of the deposition increase with applied voltage.

[0042] Samples of the treated carbon papers according to Examples 3 and4 were then sintered at 380° C. for 10 to 30 minutes and tested forplugging using a Gurley porosity measuring instrument (Model # 4110Densometer and Model # 4320 Automatic Digital Timer, Gurley PrecisionInstrument, Troy N.Y.). An comparative untreated sample was tested also.The Gurley number for the untreated carbon paper was 7.4 seconds. TheGurley number for the treated paper of Example 4 was between 8.0 and 8.4seconds. Thus, the paper was coated with minor and acceptable loss ofporosity.

[0043] Resistivity of the treated and sintered carbon papers accordingto Examples 3 and 4 was tested using a Resistance/Compression Tester,comprising a press equipped to compress a sample between twoelectrically isolated platens so as to allow simultaneous measurement ofcompression and electrical resistivity at a given pressure. FIG. 7demonstrates resistivity vs. compression data for carbon papersaccording to Example 3 (2), Example 4 (3) and a comparative untreatedpaper (1). It can be seen that the treatment according to the inventiondid not significantly compromise the electrical and physical propertiesof the carbon paper.

[0044] Advancing and receding dynamic contact angles to water weremeasured for samples according to Examples 1, 2C, 3 and 4 usingdeionized water and a Cahn DCA-322 Dynamic Contact Angle Analyzer(Thermo Cahn, Madison, Wis.). Three cycles were measured for eachsample. The cycling is an indication of the durability of thehydrophobicity for each sample. The data is reported in Table I. TABLE IWater Water Deposition Test Advancing Receding Example Voltage Cycle No.(degrees) (degrees) 1 6 1 169 69 1 6 2 114 65 1 6 3 109 63 3 15 1 170 973 15 2 162 115 3 15 3 162 113 4 30 1 180 121 4 30 2 161 130 4 30 3 137126  2C NA 1 141 0  2C NA 2 125 0  2C NA 3 112 18

[0045] This data illustrates that the carbon paper tended to beincreasingly more hydrophobic for samples treated at higher voltages.The dip-coated sample appeared hydrophobic at first but losthydrophobicity after multiple cycling.

[0046] Various modifications and alterations of this invention willbecome apparent to those skilled in the art without departing from thescope and principles of this invention, and it should be understood thatthis invention is not to be unduly limited to the illustrativeembodiments set forth hereinabove. All publications and patents areherein incorporated by reference to the same extent as if eachindividual publication or patent was specifically and individuallyindicated to be incorporated by reference.

We claim:
 1. A method of making a hydrophobic carbon fiber constructioncomprising the steps of: a) immersing a carbon fiber construction in anaqueous dispersion of a highly fluorinated polymer; b) contacting saiddispersion with a counterelectrode; and c) electrophoreticallydepositing said highly fluorinated polymer on said carbon fiberconstruction by applying electric current between said carbon fiberconstruction and said counterelectrode.
 2. The method according to claim1 wherein said highly fluorinated polymer is selected from the groupconsisting of polytetrafluoroethylene (PTFE), fluorinated ethylenepropylene (FEP), perfluoroalkyl acrylates, hexafluoropropylenecopolymers, and tetrafluoroethylene/hexafluoropropylene/vinylidenefluoride terpolymers.
 3. The method according to claim 1 wherein saidhighly fluorinated polymer is polytetrafluoroethylene (PTFE).
 4. Themethod according to claim 1 wherein said carbon fiber construction is awoven carbon fiber construction.
 5. The method according to claim 1wherein said carbon fiber construction is a non-woven carbon fiberconstruction.
 6. The method according to claim 1 wherein said step ofelectrophoretically depositing said highly fluorinated polymer has aduration of not more than 30 minutes.
 7. The method according to claim 1wherein said step of electrophoretically depositing said highlyfluorinated polymer has a duration of not more than 15 minutes.
 8. Themethod according to claim 1 wherein said electric current is applied ata voltage of between 6 and 100 volts.
 9. The method according to claim 1additionally comprising the step of: d) sintering said highlyfluorinated polymer by heating said carbon fiber construction.
 10. Thehydrophobic carbon fiber construction made according to the method ofclaim
 1. 11. The hydrophobic carbon fiber construction made according tothe method of claim
 9. 12. The hydrophobic carbon fiber constructionaccording to claim 10 which is coated with a monolayer of particles of ahighly fluorinated polymer.
 13. The hydrophobic carbon fiberconstruction according to claim 11 which is coated with a monolayer ofparticles of a highly fluorinated polymer.
 14. A hydrophobic carbonfiber construction coated with a monolayer of particles of a highlyfluorinated polymer.
 15. The hydrophobic carbon fiber constructionaccording to claim 14 wherein said highly fluorinated polymer isselected from the group consisting of polytetrafluoroethylene (PTFE),fluorinated ethylene propylene (FEP), perfluoroalkyl acrylates,hexafluoropropylene copolymers, andtetrafluoroethylene/hexafluoropropylene/vinylidene fluoride terpolymers.16. The hydrophobic carbon fiber construction according to claim 14wherein said highly fluorinated polymer is polytetrafluoroethylene(PTFE).
 17. The hydrophobic carbon fiber construction according to claim14 wherein said carbon fiber construction is a woven carbon fiberconstruction.
 18. The hydrophobic carbon fiber construction according toclaim 14 wherein said carbon fiber construction is a non-woven carbonfiber construction.
 19. The hydrophobic carbon fiber constructionaccording to claim 14 wherein said particles of a highly fluorinatedpolymer are sintered.